One conventional communications method that uses multiple antennas is, for example, the communications method known as Multiple-Input Multiple-Out (MIMO).
In multi-antenna communications, which is typically MIMO, data reception quality and/or a data communication rate (per unit time) can be improved by modulating transmission data of one or more sequences and simultaneously transmitting the respective modulated signals from different antennas by using the same frequency (common frequency).
One type of MIMO is polarized MIMO. For example, Patent Literature (PTL) 1 (Japanese Unexamined Patent Application Publication No. 2009-33268) discloses the following.
The rank of the channel matrix is improved and the stream count ensured by switching polarization surfaces of some antennas on the transmitting side and receiving side, and approximating a transfer function between an antenna using a polarization surface that is orthogonal to these polarization surfaces to 0. When the antenna configuration is 3×3 or larger, typically all antennas use vertical polarization, and it is determined to which antennas horizontal polarization should be applied to effectively improve channel matrix quality, and the polarization surfaces are switched for only specified antennas in the transceiver.